1. Field of the Invention
The present invention relates to a gear mechanism and a torque resistance mechanism, which reduce a noise of an electric power tool.
2. Description of the Related Art
In a conventional electric power tool, variations in rotation occurs owing to an excitation force by an alternating current power supply, a pitch error and an eccentricity of a gear or the like, and a surface of a gear at a drive shaft side and a surface of a gear at an output side repeat collision or vibration with each other. In this case, a noise is generated owing to the collision of the surfaces of the gears with each other, and a collision force of the surfaces of the gears fall into the excitation force. Therefore, a noise has been generated in a portion other than a gear mechanism unit. Particularly during activation, since a torque at the drive shaft side is large, an impact force thereof is also large. Accordingly, a large noise has been generated. In order to reduce the foregoing noise, a precision of the gear has been improved, or a shape of the gear of the surface has been changed, thus coping with the noise. In order to reduce a gear noise mechanically, a backlash of a steel key in an inner periphery of the gear, which is for jointing the power transmission shaft and the gear, has been utilized. Note that a configuration using the steel key transmits a torque by a shear load.
As a method for improving the precision of the gear, a method of performing gear cutting for the gear twice or more, a method of polishing the gear surface by use of a cutting sandstone instead of a cutter or the like has been performed. However, in such methods, there have been problems that productivity has been lowered owing to an increase of the number of processing steps and that a cost has been increased accompanied therewith.
Moreover, even if the precision of the gear is improved, there has been a problem of deterioration of gearing between the gears owing to an inferior assembly precision with other parts, resulting in generation of the noise.
Furthermore, in the configuration using a space between the steel key and a key groove, an impact stress applied to the key and the key groove portion of the gear during the activation has been significantly large as compared with that in a configuration in which the gear and the shaft are jointed by forcible insertion, thus causing a problem that a usable range of the configuration has been limited in terms of strength.